1. Field of the Invention
The present invention is related to a voice relaying apparatus and a voice relaying method in order to carry out a digital voice communication in an asynchronous transfer mode (ATM).
2. Description of the Related Art
Conventionally, a digital voice communication network for carrying out an inter-office communication in an asynchronous transfer mode is well known. In this digital voice communication network, a voice signal is relayed and switched in accordance with a procedure described below. At first, a cell received from a relay line is disassembled. As a consequently, a low-bit-rate coding voice signal is produced. This low-bit-rate coding voice signal is further decoded into a PCM voice signal of 64 kbps that can be treated within a digital switch and sent to the digital switch. The digital switch switches this PCM voice signal and outputs the switched PCM voice signal. The PCM voice signal outputted by the digital switch is again encoded into the low-bit-rate coding voice signal, and then assembled into the cell, and further sent to the relay line.
In the above-mentioned relaying and switching method, each time the relay switch operation is carried out by the digital switch, the processes are carried out, such as the disassembling of the cell, the decoding of the low-bit-rate coding voice signal, the encoding of the PCM voice signal and the assembling of the cell. This results in deterioration of quality of the voice sent and received through the digital voice communication network and also leads to increase of a transmission delay time of the voice.
As a first conventional technique to solve this problem, Japanese Laid-Open Patent Disclosure (JP-A-Heisei 9-98169) discloses “VOICE RELAYING AND SWITCHING SYSTEM”. In this voice relaying and switching system, a cell received from a relay line is disassembled and converted into the low-bit-rate coding voice signal. Then, a predetermined synchronous signal is added to this low-bit-rate coding voice signal to thereby generate a pseudo digital voice signal, which is sent to a switch. If a digital voice signal switched by the switch includes the predetermined synchronous signal, only the low-bit-rate coding voice signal is extracted from the digital voice signal, and then assembled into the cell, and further sent to the relay line.
Accordingly, when a connection destination of the switch is the relay line, the process for decoding the low-bit-rate coding voice signal and the process for encoding the PCM voice signal can be omitted. As a result, this case can avoid the deterioration of call quality caused by these processes and the increase of transmission delay. However, in this voice relaying and switching system, because the digital voice signal must pass through the section for disassembling the cell, the switch and the section for assembling the cell at a speed of at least 64 kbps, the hardware which operate at high speed is required.
Also, Japanese Laid-Open Patent Disclosure (JP-A-Heisei 10-4415) discloses “DATA TRANSMITTING APPARATUS”, as a second conventional technique. FIG. 1 shows the structure of this data transmitting apparatus. This data transmitting apparatus is provided with a switch (PBX) 704, a transcoder 702 and an ATM multiplexer 701. As the PBX 704, a switch that can function as a relay station is employed. The ATM multiplexer 701 is composed of a cell assembling/disassembling device (CLAD) 715 for assembling the cell in accordance with data received from a transcoder 703 and a cell assembling/disassembling device (CLAD) 714 for assembling the cell in accordance with data received from another data transmitting apparatus and then transmitting to the transcoder 702.
In this data transmitting apparatus, when the PBX 704 does not serve as the relay station, the transcoder 703 performs a band compression on the data received from the PBX 704, and sends to the cell assembling/disassembling device 715. Then, the transcoder 702 releases the band compression of the signal from the cell assembling/disassembling device 714, and sends to the PBX 704. However, when the PBX 704 serves as the relay station, the transcoder 703 does not perform the band compression of the data received from the PBX 704, and sends to the cell assembling/disassembling device 715. The transcoder 702 does not release the band compression of the data from the cell assembling/disassembling device 714, and sends to the PBX 704. Hence, when the PBX 704 serves as the relay station, the process for disassembling the cell and the process for assembling the cell can be omitted in the relay station to thereby avoid the deterioration of the call quality and the increase of the transmission delay caused by these processes, even in a case of a multiple-stage relay.
Moreover, Japanese Laid-Open Patent Disclosure (JP-A-Heisei 9-55753) discloses “METHOD FOR RELAYING AND SWITCHING COMPRESSED VOICE IN ATM”, as a third conventional technique. In this method for relaying and switching a compressed voice in ATM, it is detected whether or not a cell (digital compression voice data) received from an ATM network is relayed and switched by a digital switch, when it is relayed and switched by the digital switch and again transmitted to the ATM network. When a cell (digital compression voice data) is received from the ATM network, such a check is done that whether or not a relay switch operation is accomplished by the digital switch. Then, if such a fact that the relay switch operation is accomplished is detected, the cell is passed without the compression and expansion of the voice data and the assembling and disassembling of the cell.